It has become recognized that telephone service is a target of theft. If a telephone line is infrequently used, a thief can attach an unauthorized telephone receiver between the ring and tip lines which extend between an off premises telephone central office and the premises of a telephone user. Thereafter the thief can make use of the stolen telephone line at will. When an authorized telephone handset is put in use, the thief can terminate any call in process and reinitiate it after the authorized user has completed his call. Typically, when the authorized user finds an unauthorized party on his line, he assumes that it is the result of a temporary malfunction of equipment in the central telephone office, and it is not until he receives the next monthly bill that he learns of the theft. In the meantime, the thief will disconnect the intruding telephone unit before its point of attachment can be found.
Existing methods to prevent such theft generally do not protect the most vulnerable portions of the telephone circuit. For example, Perry, U.S. Pat. No. 5,086,459 is directed to an alternate wiring of a telephone interface which would prevent a theft of service by one who is not familiar with the wiring of Perry and attempts to attach an illegal telephone unit to the subscriber's interface. In Wright, U.S. Pat. No. 5,003,586 a protective device is also incorporated into the telephone interface. In Wright, the black and yellow, alternate wires of a RJ11 plug from an authorized telephone system are connected together, and the joined black and yellow wires from the authorized telephone unit will short out an interrupting circuit in the interface thereby permitting use of the telephone service system. Where the theft of the telephone service occurs between the interface and the telephone central office, however, the device of neither Wright nor Perry will provide the protection needed.
Genin, U.S. Pat. No. 4,885,768 discloses a device which is attached between the ring and tip lines of a telephone line and detects a change in the voltage between the lines. The device of Genin is connected to an active dipole filter to prohibit either a pulse dialing, or dual tone multi frequency dialing. The device, however, is deactivated when it recognizes a code inserted by an authorized user. Although the device of Genin will prevent an unauthorized user from initiating a telephone call, a user of an authorized telephone can not initiate a call without first inserting the code, which is an inconvenience to the user of an authorized telephone unit. Furthermore, the device of Genin can be avoided by an unauthorized user who somehow acquires the code. Newell, U.S. Pat. No. 4,953,202 discloses a device similar to Genin, and also requires the use of a code.
None of the existing devices, therefore, prohibit the use of an unauthorized telephone unit between the source of an off premises telephone central office and the authorized telephone units without requiring the use of codes such as disclosed by Newell and Genin.